One of the principal commercial processes employed to manufacture high density linear polyethylene is to polymerize ethylene in the presence of a chromium catalyst supported on silica. The active chromium catalyst may be chromium oxide or certain complex organic chromium compounds such as dicyclopentadienyl chromium II.
In a specific ethylene polymerization process, the polymerization is carried out in a hydrocarbon medium having little or no solvent action on the resin being produced, and the resin, as formed, precipitates as fine solid particles. For this reason, this particular process is known in the art as the Particle Form Process. As used throughout this specification, the term Particle Form Process will be restricted to a process carried out in the presence of a chromium catalyst supported upon silica and carried out in a hydrocarbon medium having solubility characteristics such that the resin, as produced, precipitates in the form of fine solid particles.
One of the limitations of the particle Form Process is that the resin produced by the process has a very high molecular weight and a correspondingly low melt index. For many purposes, the art desires polyethylene resins of lower molecular weight and correspondingly higher melt index. It obviously would be desirable to be able to operate the Particle Form Process under varying conditions such that the resin produced would have a preselected molecular weight and melt index, which molecular weight and melt index could be varied over a wide range.
Many workers have attempted to modify the Particle Form Process to expand its capability to manufacture ethylene polymers having lower molecular weights and higher melt indexes. Such efforts have been directed principally to modifying the properties of the chromium supported catalysts employed in the process. The success of such efforts has been marginal, at best, and many workers in the art believe that the Particle Form Process inherently is restricted to the manufacture of high molecular weight -- low melt index resins.